1. Field of the Invention
The invention relates to a steering system.
2. Description of the Related Art
A steering system has been available in which a driver's steering operation is assisted by transmitting a rotary motion of a motor to a ball screw mechanism via a toothed belt and a pulley to convert the rotary motion into a linear motion of a rack shaft. When tension of the belt decreases to loosen the belt, teeth of the belt may be offset from corresponding teeth of the pulley to cause improper meshing when an increased torque is transmitted between the belt and the pulley. This precludes the large torque from being transmitted. Thus, the steering apparatus is provided with a belt tension adjusting mechanism that adjusts the tension of the belt. The belt tension adjusting mechanism disclosed in Japanese Patent Application Publication No. 2007-112245 (JP 2007-112245 A) has a seat supported by a housing via a pin so as to be able to swing, a tension pulley rotatably supported by the seat so as to contact the belt, and a feed screw mechanism that presses the seat so as to press the tension pulley against the belt. An amount by which a screw in the feed screw mechanism is fed is adjusted to adjust an amount by which the tension pulley is pressed against the belt.
The belt is elongated due to aging, and thus, when a given period elapses, the belt has a lower tension than the belt in an initial state. Thus, the tension of the belt in the initial state needs to be set with an expected decrease in tension of the belt taken into account. To allow the tension of the belt to exceed a prescribed value when the given period elapses, the tension pulley needs to be pressed hard against the belt such that the belt in the initial state has a tension higher than the prescribed value. The amount by which the feed screw mechanism feeds the screw increases as a force at which the tension pulley is pressed against the belt increases. However, for example, a heavier load is imposed on a bearing that supports a nut such that the nut is rotatable as the seat is pressed harder by increasing the amount by which the screw is fed. Thus, a load-carrying capacity for the load on the bearing needs to be enhanced.